Sound generating device

ABSTRACT

A driving device (14) of an acoustic vibration plate (11) is arranged between a speaker frame (10) and the acoustic vibration plate (11). The driving device (14) is comprised of a pair of piezoelectric vibration plates (1, 2) arranged facing each other across a certain distance. The outer peripheries of the piezoelectric vibration plates (1, 2) are connected to each other by an annular spacer (7). When a drive signal is applied to the piezoelectric vibration plates (1, 2), the piezoelectric vibration plates (1, 2) repeatedly undergo flexing motion wherein their centers flex alternately in opposite directions. At this time, the flexing directions of the piezoelectric vibration plates (1, 2) are always reverse to each other.

TECHNICAL FIELD

The present invention relates to a sound generating device.

BACKGROUND ART

As piezoelectric vibration plates, there are known unimorphs comprisedof a disk-shaped thin metal plate with a piezoelectric ceramic layerformed on just one side and bimorphs comprised of a disk-shaped thinmetal plate with piezoelectric ceramic layers formed on both sides.These unimorph and bimorph type piezoelectric vibration plates performflexing oscillation wherein the centers of the piezoelectric vibrationplates alternately flex in opposite directions when the voltage appliedto the piezoelectric ceramic layers is changed. Known in the art is aspeaker which uses such a flexing oscillation of a piezoelectricvibration plate to generate sound. In such a conventional speaker, theperipheral portion of the piezoelectric vibration plate was usuallysupported by the frame of the speaker, the center of the piezoelectricvibration plate was connected to an acoustic vibration plate, and theacoustic vibration plate was made to oscillate by the piezoelectricvibration plate so as to produce sound from the acoustic vibration plate(for example, see Japanese Unexamined Patent Publication (Kokai) No.60-182300).

This piezoelectric vibration plate, however, has a high naturalfrequency and a high Q value at the resonance point and has the propertyof a reduction in the sound pressure level along with a fall in thefrequency. Accordingly, there is the problem that when, like in thepast, the oscillation of the piezoelectric vibration plate is merelydirectly transmitted to the acoustic vibration plate as it is, the soundbecomes distorted and the bass sound pressure level is insufficient atthe resonance point.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a sound generatingdevice which is able to give a sufficiently high sound pressure leveleven in the bass region.

According to the present invention, there is provided a sound generatingdevice provided with a driving device having a plurality ofpiezoelectric vibration plates which are arranged spaced from each otherin an axial direction, wherein either of the peripheral portions orcenters of adjacent piezoelectric vibration plates are connected to eachother and adjacent piezoelectric vibration plates are made to flex inopposite directions from each other, the piezoelectric vibration platepositioned at one end of said plurality of piezoelectric vibrationplates being connected to an acoustic vibration plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a type I module,

FIG. 2 is a front view of the module shown in FIG. 1,

FIGS. 3(A), 3(B) are views for explaining the operation of the moduleshown in FIG. 1,

FIG. 4 is a cross-sectional side view of a type II module,

FIG. 5 is perspective view of the module shown in FIG. 4,

FIGS. 6(A), 6(B) are views for explaining the operation of the moduleshown in FIG. 4,

FIG. 7 is view showing various driving devices,

FIG. 8 is a cross-sectional side view of a speaker using the type Imodule shown in FIG. 1,

FIG. 9 is a partially enlarged cross-sectional side view of FIG. 8,

FIG. 10 is a cross-sectional side view of part of a speaker showinganother embodiment,

FIG. 11 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 12 is a cross-sectional side view of a speaker using the type IIspeaker shown in FIG. 4,

FIG. 13 is a partially enlarged cross-sectional side view of FIG. 12,

FIG. 14 is a perspective view of the type II module,

FIG. 15 is a cross-sectional side view of part of a speaker showinganother embodiment,

FIG. 16 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 17 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 18 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 19 is a cross-sectional side view of part of a speaker showing amodification of FIG. 18,

FIG. 20 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 21 is a cross-sectional side view of part of a speaker showingstill another embodiment,

FIG. 22 is a partially enlarged cross-sectional side view of FIG. 21,

FIG. 23 is a view showing the relationship of the frequency f and thesound pressure level P,

FIG. 24 is a front view of a speaker showing another embodiment, and

FIG. 25 is a cross-sectional view seen along line XXV--XXV of FIG. 24.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 and FIG. 2 show an example of a driving device for driving anacoustic vibration plate of a sound generating device. Referring to FIG.1 and FIG. 2, the driving device is comprised of a pair of disk-shapedmetal piezoelectric vibration plates 1 and 2 arranged facing each otheracross a certain distance in the axial direction. The centers of thesepiezoelectric vibration plates 1 and 2 are connected to each other by ametal or plastic connecting rod 3. On the two sides of each of thepiezoelectric vibration plates 1 and 2 are formed annular piezoelectricceramic layers 4. Accordingly, in the example shown in FIG. 1 and FIG.2, the piezoelectric vibration plates 1 and 2 are comprised of bimorphs.

FIG. 1 shows the directions of polarization of the piezoelectric ceramiclayers 4 of the piezoelectric vibration plates 1 and 2 by the arrows K.As shown in FIG. 1, in the example shown in FIG. 1, the piezoelectricvibration plates 1 and 2 are connected by the connecting rod 3 so thatthe direction of polarization K of the piezoelectric ceramic layers 4 ofone piezoelectric vibration plate 1 becomes reverse from the directionof polarization K of the piezoelectric ceramic layers 4 of the otherpiezoelectric vibration plate 2. The piezoelectric vibration plate 2 isfor example grounded through a lead wire 5. An identical driving voltageis applied through the lead wires 6 to thin film electrodes formed onthe surfaces of the piezoelectric ceramic layers 4.

When a voltage is applied to the thin film electrodes of thepiezoelectric ceramic layers 4 of the piezoelectric vibration plates 1and 2, the piezoelectric ceramic layers 4 formed on one side of thepiezoelectric vibration plates 1 and 2 extend in the radial direction,while the piezoelectric ceramic layers 4 formed on the other sidecontract. As a result, the piezoelectric vibration plates 1 and 2 flex.In the example shown in FIG. 1, as mentioned above, the directions ofpolarization K of the piezoelectric ceramic layers 4 of thepiezoelectric vibration plates 1 and 2 are reverse from each other. Inthis case, when a positive voltage and negative voltage are alternatelyapplied through the lead wires 6 to the thin film electrodes of thepiezoelectric ceramic layers 4, the piezoelectric vibration plates 1 and2 flex in reverse directions from each other as shown in FIG. 3(A) and(B). That is, the state of the piezoelectric vibration plates 1 and 2flexing outward to form a convex shape as shown in FIG. 3(A) and thestate of the piezoelectric vibration plates 1 and 2 flexing inward toform a concave shape as shown in FIG. 3(B) are alternately repeated.

In this case, if the distance between the peripheral portions of thepiezoelectric vibration plates 1 and 2 in the state shown in FIG. 3(A)is made S₁ and the distance between the peripheral portions of thepiezoelectric vibration plates 1 and 2 in the state shown in FIG. 3(B)is made S₂, then amount of displacement ΔS of the peripheral portions ofthe piezoelectric vibration plates 1 and 2 becomes ΔS=S₂ -S₁.Accordingly, if the amount of displacement is used as the output of thedriving device, the stroke of the output of the driving device becomesΔS (=S₂ -S₁). This stroke is double the stroke obtained when using asingle piezoelectric vibration plate. Accordingly, in the driving deviceshown in FIG. 1, it is possible to produce an output of the strokedouble that in the case of use of a single piezoelectric vibrationplate.

In this way, it is possible to increase the output stroke by use of apair of piezoelectric vibration plates 1 and 2. The pair ofpiezoelectric vibration plates 1 and 2 shown in FIG. 1 in this caseshows the smallest unit of combination of piezoelectric vibration platesenabling increase of the output stroke. This smallest unit ofcombination is referred to as a "module". Note that the module obtainedby connecting the centers of the pair of the piezoelectric vibrationplates 1 and 2 as shown in FIG. 1 is referred to below as a "type Imodule".

FIG. 4 and FIG. 5 show a module of a construction different from themodule shown in FIG. 1. Note that in FIG. 4 and FIG. 5, constituentelements similar to those of FIG. 1 are shown by the same referencenumerals.

Referring to FIG. 4 and FIG. 5, the outer peripheries of the pair ofpiezoelectric vibration plates 1 and 2 are affixed to a metal annularspacer 7 extending along the outer peripheries of the piezoelectricvibration plates 1 and 2. Accordingly, in the example shown in FIG. 4and FIG. 5, the pair of piezoelectric vibration plates 1 and 2 areconnected to each other through the annular spacer 7. In the exampleshown in FIG. 4 and FIG. 5 as well, the direction of polarization K ofthe piezoelectric ceramic layers 4 of one piezoelectric vibration plate1 is reverse to that of the direction of polarization K of thepiezoelectric ceramic layers 4 of the other piezoelectric vibrationplate 2 and an identical drive voltage is applied through the lead wires6 to the thin film electrodes of the piezoelectric ceramic layers 4.Accordingly, in this case too, when a positive voltage and negativevoltage are alternately applied to the thin film electrodes of thepiezoelectric ceramic layers 4, the piezoelectric vibration plates 1 and2 flex alternately in reverse directions as shown by FIG. 6(A) and 6(B).

In this case, if the distance between the centers of the piezoelectricvibration plates 1 and 2 in the state shown in FIG. 6(A) is made S₁ andthe distance between the centers of the piezoelectric vibration plates 1and 2 in the state shown in FIG. 6(B) is made S₂, then the amount ofdisplacement ΔS of the centers of the piezoelectric vibration plates 1and 2 becomes ΔS=S₂ -S₁. Accordingly, if the amount of displacement isused as the output of the driving device, the stroke of the output ofthe driving device becomes ΔS (=S₂ -S₁). This stroke is double thestroke obtained when using a single piezoelectric vibration plate.Accordingly, in the driving device shown in FIG. 4, it is possible toproduce an output of the stroke double that in the case of use of asingle piezoelectric vibration plate. Note that the module obtained byconnecting the peripheral portions of the pair of the piezoelectricvibration plates 1 and 2 as shown in FIG. 4 is referred to below as a"type II module".

The representative modules in the case of use of a pair of piezoelectricvibration plates 1 and 2 are the type I module and type II moduleexplained above. These modules may be used as the basis for preparingdriving devices comprised of various combinations of three or morepiezoelectric vibration plates. Typical examples of these drivingdevices are shown in FIG. 7. Note that in FIG. 7, the driving devicesshown in the column with two piezoelectric vibration plates are the typeI module and type II module explained above.

Referring to FIG. 7, the driving devices comprised of combinations ofthree piezoelectric vibration plates are shown as type III and type IV.The driving device shown by type III is a combination of the type IImodule with a single piezoelectric vibration plate 8 and is formed byconnecting the center of the piezoelectric vibration plate 2constituting part of the type II module with the center of the singlepiezoelectric vibration plate 8 by a connecting rod 3. In this drivingdevice, when a drive voltage is applied, the piezoelectric vibrationplate 2 and the piezoelectric vibration plate 8 flex in reversedirections from each other. Therefore, this driving device gives anoutput stroke three times the output stroke when using a singlepiezoelectric vibration plate.

The driving device shown by type IV is also a combination of a type IImodule and a single piezoelectric vibration plate 9 and is formed byconnecting the center of the piezoelectric vibration plate 1constituting part of the type II module and the center of the singlepiezoelectric vibration plate 9 by a connecting rod 3. In this drivingdevice as well, when a drive voltage is applied, the piezoelectricvibration plate 1 and the piezoelectric vibration plate 9 flex inreverse directions from each other and therefore this driving devicealso gives an output stroke three times the output stroke when using asingle piezoelectric vibration plate.

On the other hand, as shown in FIG. 7, the driving devices comprisingcombinations of four piezoelectric vibration plates are shown as type Vand type VI. The driving device shown by type V is a combination of thetype II module and two piezoelectric vibration plates 8 and 9. Seen inanother way, a type II module is inserted between the pair ofpiezoelectric vibration plates 1 and 2 of a type I module. That is, thisdriving device is formed by connecting the center of one piezoelectricvibration plate 1 constituting part of the type II module and the centerof the piezoelectric vibration plate 9 by a connecting rod 3 andconnecting the other piezoelectric vibration plate 2 constituting thetype II module and the center of the piezoelectric vibration plate 8 bya connecting rod 3. In this driving device, when a drive voltage isapplied, the piezoelectric vibration plate 1 and the piezoelectricvibration plate 9 flex in opposite directions to each other and thepiezoelectric vibration plate 2 and the piezoelectric vibration plate 8flex in opposite directions to each other, so an output stroke fourtimes the output stroke when using a single piezoelectric vibrationplate can be obtained.

On the other hand, the driving device shown by type VI is a combinationof two type II modules and is formed by connecting the centers of thepiezoelectric vibration plates 1 and 2 of the modules facing each otherby a connecting rod 3. In this driving device too, it is possible toobtain a stroke four times the output stroke when using a singlepiezoelectric vibration plate.

Further, as shown in FIG. 7, driving devices comprising combinations offive piezoelectric vibration plates are shown as type VII and type VIII.Driving devices comprising combinations of six piezoelectric vibrationplates are shown as type IX and type X. The configurations of the typeVII, VIII, IX, and X driving devices are clear from FIG. 7, so noparticular explanation will be made, but in each of the driving devicesof the type VII, VIII, IX, and X, the mutually adjoining piezoelectricvibration plates 1, 2, 8, and 9 flex in mutually opposite directionswhen a drive voltage is applied. Accordingly, in the type VII and VIIIdriving devices, an output stroke five times the output stroke whenusing a single piezoelectric vibration plate is obtained, while in thetype IX and X driving devices, an output stroke six times the outputstroke when using a single piezoelectric vibration plate is obtained.Note that while not shown in FIG. 7, it is possible to form a drivingdevice comprising seven or more piezoelectric vibration plates in asimilar way.

Next, an explanation will be made of representative examples of thesound generating device using the driving devices shown in FIG. 7 todrive the acoustic vibration plate.

FIG. 8 and FIG. 9 show the case of application of the present inventionto a speaker and use of the type I module shown in FIG. 1 as a drivingdevice for the speaker.

Referring to FIG. 8 and FIG. 9, reference numeral 10 shows a speakerframe and 11 an acoustic vibration plate. The outer periphery of theacoustic vibration plate 11 is bonded to the outer periphery of thespeaker frame 10. Further, a packing 11a is bonded on the outerperiphery of the acoustic vibration plate 11. In the embodiment shown inFIG. 8 and FIG. 9, the acoustic vibration plate 11 is formed from conepaper, but the acoustic vibration plate 11 may be formed from wood,plastic, or a thin metal sheet. The inner periphery of the acousticvibration plate 11 is connected to the outer periphery of one of thepiezoelectric vibration plates 1 of the driving device 12, while theouter periphery of the other piezoelectric vibration plate 1 of thedriving device 12 is connected to the speaker frame 10.

As explained at the start, a piezoelectric vibration plate has a highnatural frequency and the sound pressure level falls as this frequencyfalls. In the embodiment shown in FIG. 8 and FIG. 8, however, the drivestroke given by the driving device 12 to the acoustic vibration plate 11becomes twice that when using a single acoustic vibration plate, so theamplitude of the acoustic vibration plate 11 becomes larger even in thelow frequency region and therefore the bass sound pressure level can bemade higher.

Further, if the pair of piezoelectric vibration plates 1 and 2 areconnected with each other by a connecting rod 3, the natural frequencyof the driving device 12 becomes considerably lower than the naturalfrequency of the piezoelectric vibration plates and as a result theresonance point moves to the low frequency side. Accordingly, from thisviewpoint as well, the amplitude of the acoustic vibration plate at thelow frequency region can be made larger and therefore the bass soundpressure level can be raised much higher.

FIG. 10 shows another embodiment. As shown in FIG. 10, in thisembodiment, to lower the natural frequency of the driving device 13 andfurther to make the sound pressure level flat over a wide frequencyregion, an annular elastic member 13 comprised of rubber is attached tothe outer periphery of the piezoelectric vibration plate 2. That is, asshown in FIG. 10, since the elastic member 13 has a relatively largemass, it is possible to further reduce the natural frequency of thedriving device 13 and therefore to further raise even the bass soundpressure level. Further, if the natural frequency of the driving device13 is reduced, the resonance point appears at the bass region, but theelastic member 13 functions to reduce the Q value at this resonancepoint and to reduce the Q value at the next higher resonance pointappearing in the high frequency region.

That is, since the elastic member 13 has a relatively large mass asexplained above, this elastic member 13 acts to suppress the movement ofthe peripheral portion of the piezoelectric vibration plate 2 in theforward-reverse direction due to its inertia. Accordingly, as shown inFIG. 10, even if the elastic member 13 is not supported by the speakerframe 10, the acoustic vibration plate 11 is made to oscillate when thepiezoelectric vibration plates 1 and 2 engage in flexing motion. Whenthe speed of flexing motion of the piezoelectric vibration plate 2 isslow, however, that is, in the low frequency region, the elastic member13 moves as a whole in accordance with the movement of the peripheralportion of the piezoelectric vibration plate 2. As opposed to this, whenthe speed of flexing motion of the piezoelectric vibration plate 2 isfast, that is, in the high frequency region, the elastic member 13 as awhole cannot follow the movement of the peripheral portion of thepiezoelectric vibration plate 2 and therefore the movement of the outerperiphery of the elastic member 13 lags behind the movement of the innerperiphery of the elastic member 13. As a result, the elastic member 13deforms. This deformation motion is repeated.

This deformation of the elastic member 13 occurs due to the vibrationenergy and therefore the larger the amount of deformation of the elasticmember 13, the greater the vibration energy consumed to cause thedeformation of the elastic member 13. In other words, the larger theamount of deformation of the elastic member 13, the greater thevibration energy absorbed by the elastic member 13. However, asexplained above, the amount of deformation of the elastic member 13becomes greater the higher the frequency. Accordingly, as shown in FIG.10, if the elastic member 13 is attached to the piezoelectric vibrationplate 2, it becomes possible to attenuate the high frequency vibrationby this elastic member 13. As a result, it is possible to relativelyincrease the amplitude of the low frequency region and therefore toraise the bass sound pressure level.

On the other hand, at the resonance point, not only does the amplitudebecome larger, but also the speed of flexing motion of the piezoelectricvibration plate 2 becomes faster and therefore the oscillation at theresonance point is attenuated by the elastic member 13. Accordingly, ifthe elastic member 13 is attached to the piezoelectric vibration plate2, the Q value becomes smaller and therefore the sound pressure levelcan be made flat over a wide frequency region.

FIG. 11 shows still another embodiment. In this embodiment, the outerperiphery of the annular elastic member 13 is affixed to the speakerframe 10. If the outer periphery of the elastic member 13 is affixed tothe speaker frame 10 in this way, the amount of deformation of theelastic member 13 at the time of occurrence of high frequencyoscillation becomes further greater and therefore it becomes possible tofurther attenuate the high frequency oscillation and possible to furtherreduce the Q value. Also, if the outer periphery of the elastic member13 is affixed to the speaker frame 10, it becomes possible to greatlysuppress the amount of movement of the outer periphery of thepiezoelectric vibration plate 2 in the forward-reverse direction at thetime of occurrence of low frequency oscillation. As a result, it ispossible to increase the amplitude of the acoustic vibration plate 11 atthe low frequency region and therefore to increase the bass soundpressure level.

FIG. 12 to FIG. 14 show the case of use of the module of the type IIshown in FIG. 4 as the driving device of a speaker.

Referring to FIG. 12 and FIG. 13, the driving device 14 comprised of amodule of type II is arranged between the acoustic vibration plate 11and the speaker frame 10. The center of one piezoelectric vibrationplate 1 constituting the type II module is connected to the center ofthe acoustic vibration plate 11 through a metal or plastic connectingrod 3a by for example a nut 15, while the center of the otherpiezoelectric vibration plate 2 constituting the type II module isconnected to the speaker frame 10 through a metal or plastic connectingrod 3b by for example the nut 16. In this embodiment too, the drivestroke given by the driving device 14 to the acoustic vibration plate 11becomes double that when using a single piezoelectric vibration plate,so the amplitude of the acoustic vibration plate 11 becomes larger evenin the low frequency region and therefore it is possible to raise thebass sound pressure level.

Further, when the pair of piezoelectric vibration plates 1 and 2 areconnected to each other by an annular spacer 7 as in this embodiment,the natural frequency of the driving device 14 becomes considerablylower than the natural frequency of the piezoelectric vibration platesand, as a result, the resonance point moves to the low frequency side.Accordingly, from this viewpoint as well, it is possible to increase theamplitude of the acoustic vibration plate 11 in the low frequency regionand therefore the bass sound pressure level is raised much higher.Further, in this embodiment, to make the natural frequency of thedriving device 13 lower and to make the sound pressure level flat over awide frequency region, a plurality of communicating holes 17 are formedin the annular spacer 7 and an air damper chamber 18 communicatingthrough these communicating holes 17 to the outside air is formedbetween the pair of piezoelectric vibration plates.

If the air damper chamber 18 increases in volume due to the flexingmotion of the piezoelectric vibration plates 1 and 2, outside air flowsthrough the communicating holes 17 into the air damper chamber 18, whileif the air damper chamber 18 decreases in volume, air in the air damperchamber 18 flows out to the outside air through the communicating holes17. In this case, time is required for the inflowing and outflowingaction of the air through the communicating holes 17, so the faster thespeed of flexing motion of the piezoelectric vibration plates 1 and 2,that is, the higher the frequency of vibration, the harder it becomesfor the piezoelectric vibration plates 1 and 2 to flex. That is, if thepiezoelectric vibration plates 1 and 2 attempt to flex outward to give aconvex shape as shown in FIG. 6(B), the pressure inside the air damperchamber 18 falls, so the flexing motion of the piezoelectric vibrationplates 1 and 2 is suppressed, while when the piezoelectric vibrationplates 1 and 2 attempt to flex inward to give a concave shape as shownin FIG. 6(A), the pressure inside the air damper chamber 18 rises, sothe flexing motion of the piezoelectric vibration plates 1 and 2 issuppressed. In this way, due to the damper action of the air damperchamber 18, the faster the speed of flexing motion of the piezoelectricvibration plates 1 and 2, the more the flexing motion of thepiezoelectric vibration plates 1 and 2 is suppressed. In other words,the faster the speed of flexing motion of the piezoelectric vibrationplates 1 and 2, that is, the higher the frequency of oscillation, themore the oscillation of the piezoelectric vibration plates 1 and 2 issuppressed. Accordingly, by providing such an air damper chamber 18, itis possible to relatively increase the bass sound pressure level andfurther reduce the Q value at the resonance point, so it is possible tomake the sound pressure level flat over a wide frequency region.

FIG. 15 shows still another embodiment. In this embodiment, the annularspacer 19 connecting the peripheral portions of the piezoelectricvibration plates 1 and 2 together is formed from an elastic member suchas rubber and a plurality of communicating holes 20 communicating theair damper chamber 18 with the outside air are formed in the peripheralportions of the piezoelectric vibration plates 1 and 2. Accordingly, inthis embodiment too, it is possible to relatively raise the bass soundpressure level by the attenuation action of the air damper chamber 18 onthe high frequency oscillation and possible to make the sound pressurelevel flat over a wide frequency region. Further, in this embodiment,the higher the frequency, the greater the frequency of deformation ofthe elastic member 19, so the higher the frequency, the greater theamount of absorption of oscillation by the elastic member 19.Accordingly, in this embodiment, it is possible to further cause thehigh frequency oscillation to attenuate.

FIG. 16 shows still another embodiment. Referring to FIG. 16, in thisembodiment, the center of the elastic plate 21 comprised of rubber isconnected to the center of the piezoelectric vibration plate 2 through aconnecting rod 3b by a nut 16. This elastic plate 21 acts in a similarway as the elastic member 13 shown in FIG. 10.

That is, the elastic plate 21 has a relatively large mass and thereforethe elastic plate 21 acts to suppress the movement of the center of thepiezoelectric vibration plate 2 in the forward-reverse direction by itsinertia. Accordingly, as shown in FIG. 16, even if the elastic plate 21is not supported by the speaker frame 10, the acoustic vibration plate11 is made to vibrate when the piezoelectric vibration plates 1 and 2engage in flexing motion. On the other hand, when the speed of flexingmotion of the piezoelectric vibration plates 1 and 2 is slow, that is,in the low frequency region, the elastic body 21 moves as a whole inaccordance with the movement of the center of the piezoelectricvibration plate 2. As opposed to this, when the speed of flexing motionof the piezoelectric vibration plates 1 and 2 is fast, that is, in thehigh frequency region, the elastic body 21 as a whole cannot follow themovement of the center of the piezoelectric vibration plate 2 andtherefore the movement of the outer periphery of the elastic body 21lags behind the movement of the center of the elastic body 21. As aresult, the elastic body 21 deforms and this deformation motion isrepeated.

In this case, however, the larger the amount of deformation of theelastic plate 21, the larger the vibration energy absorbed by theelastic plate 21. The amount of deformation of the elastic plate 21shown in FIG. 16 becomes larger the higher the frequency. Accordingly,as shown in FIG. 16, if the elastic plate 21 is attached to thepiezoelectric vibration plate 2, it is possible to cause the highfrequency oscillation to attenuate by the elastic plate 21. As a result,it becomes possible to relatively increase the amplitude at the lowfrequency region and therefore to raise the bass sound pressure level.

Further, as explained above, not only does the amplitude become largerat the resonance point, but also the speed of the flexing motion of thepiezoelectric vibration plates 1 and 2 increases. Therefore, theoscillation at the resonance point is attenuated by the elastic plate21. Accordingly, if the elastic plate 21 is attached to thepiezoelectric vibration plate 2, the Q value becomes smaller andtherefore it is possible to make the sound pressure level flat over awide frequency region.

FIG. 17 shows still another embodiment. In this embodiment, the outerperiphery of the elastic plate 21 is affixed to the speaker frame 10. Ifthe outer periphery of the elastic plate 21 is affixed to the speakerframe 10 in this way, the amount of deformation of the elastic plate 21at the time of occurrence of high frequency oscillation becomes largerand therefore it is possible to further cause the high frequencyoscillation to attenuate and further cause the Q value to fall. Also, ifthe outer periphery of the elastic plate 21 is affixed to the speakerframe 10, the amount of movement of the center of the piezoelectricvibration plate 2 in the forward-reverse direction at the time ofoccurrence of low frequency oscillation on can be suppressed to a largeextent. As a result, it is possible to increase the amplitude of theacoustic vibration plate 11 in the low frequency region and therefore toincrease the bass sound pressure level.

Up to now, the present invention has been explained in relation to thecase of application to a driving device 12 comprised of type I moduleand a driving device 14 comprised of the type II module, but theconstructions of the embodiments explained up to here may be applied tothe various driving devices of the constructions shown as type III totype X in FIG. 7. Below, an explanation will be made of typical examplesof application of the constructions of the embodiments explained aboveto driving devices of the constructions shown as type III to type X.

FIG. 18 shows the case of use of the driving device of the type VI shownin FIG. 7 as the driving device of a speaker. That is, in the embodimentshown in FIG. 18, the driving device 22 has a construction of twomodules of type II shown in FIG. 4 connected in series. The centers ofthe two piezoelectric vibration plates positioned at the center amongthe four piezoelectric vibration plates 1 and 2 are connected to eachother by a connecting rod 3c. In this embodiment, as explained above, itis possible to obtain an output stroke four times that of the case ofuse of a single piezoelectric vibration plate as mentioned above.

FIG. 19 shows a modification of the driving device 22 shown in FIG. 18.In this modification, the centers of the two piezoelectric vibrationplates 1 and 2 positioned at the center among the four piezoelectricvibration plates 1 and 2 are connected by a hollow sleeve 23.Accordingly, in this embodiment, the air damper chambers 18 formed inthe modules are communicated with each other through the hollow sleeve23.

FIG. 20 shows the case of application of a construction in which use ismade of a driving device of the type III shown in FIG. 7 as the drivingdevice of the speaker and use is made of the annular elastic member 13shown in FIG. 11 to cause attenuation of the high frequency oscillationof the driving device 24. That is, in the driving device 24, the centerof the piezoelectric vibration plate 2 constituting part of the type IImodule and the center of the single piezoelectric vibration plate 8 areconnected to each other through a connecting rod 3b. The peripheralportion of the single piezoelectric vibration plate 8 is connected tothe speaker frame 10 through an annular elastic member 13 comprised of arubber.

FIG. 21 and FIG. 22 show the case of application of a construction inwhich use is made of a driving device of the type V shown in FIG. 7 asthe driving device of the speaker and use is made of the annular elasticmember 13 shown in FIG. 11 to cause attenuation of the high frequencyvibration of the driving device 25. That is, in the driving device 25,the center of the piezoelectric vibration plate 2 constituting part ofthe type II module and the center of the single piezoelectric vibrationplate 8 are connected to each other through a connecting rod 3b by abolt 26 and nut 16. The peripheral portion of the single piezoelectricvibration plate 8 is connected to the speaker frame 10 through anannular elastic member 13 comprised of a rubber. Further, in thisdriving device 25, the center of the piezoelectric vibration plate 1constituting part of the type II module and the center of the singlepiezoelectric vibration plate 9 are connected to each other through thehollow sleeve 27. The outer periphery of the single piezoelectricvibration plate 9 is connected to the inner periphery of the acousticvibration plate 11.

Further, in this driving device 25, the front end of the hollow sleeve27 opens to the outside. The opening of the hollow sleeve 27 is closedoff by a plug 28 made of a plastic material, for example. The plug 28 isnot inserted before assembling the driving device 25. When the drivingdevice 25 is being assembled, the plug 28 is inserted into the openingof the hollow sleeve 27 after bolting the piezoelectric vibration plates2 and 8. This forms an air damper chamber 18 between the piezoelectricvibration plates 1 and 2. In this driving device 25, further, adiaphragm 29 is attached to cover the single piezoelectric vibrationplate 9.

With this driving device 25, an output stroke four times the case of useof a single piezoelectric vibration plate is obtained. Further, withthis driving device 25, the resonance frequency of the driving device 25becomes much smaller and, further, the high frequency oscillation isattenuated to a large degree by the high frequency attenuation action ofthe air damper chamber 18 and the high frequency attenuation action ofthe elastic member 13, so the Q value is made much lower. As a result,it is possible to maintain a high sound pressure level overall and toobtain a flat sound pressure level over a wide frequency region.

FIG. 23 shows the results of an experiment investigating therelationship between the frequency f and the sound pressure level P. InFIG. 23, A shows a speaker of the construction shown in FIG. 12, while Bshows a speaker of the construction shown in FIG. 21. Note that FIG. 23shows the case of application of a drive voltage giving a substantiallyequal sound pressure level P at a frequency f of 1000 Hz to the drivingdevices 14 and 25. From FIG. 23, it is learned that the speaker of theconstruction shown in FIG. 21 has a flat sound pressure level P across awide frequency region.

FIG. 24 and FIG. 25 show still another embodiment. Referring to FIG. 24and FIG. 25, reference numeral 30 shows a speaker frame and 31 shows anacoustic vibration plate. In this embodiment, a plurality of drivingdevices 22 of the type VI in FIG. 7 are arranged in parallel between thespeaker frame 30 and the acoustic vibration plate 31. Accordingly, inthis embodiment, the acoustic vibration plate 31 is simultaneouslydriven by a plurality of driving devices 22. Note that in this case, itis possible to use any of the types of driving devices shown in FIG. 7.

The speaker using the piezoelectric vibration plates of the presentinvention not only has the advantage of being much lighter in weightcompared with the conventional dynamic speakers, but also has theadvantage of not requiring the use of permanent magnets as in dynamicspeakers and therefore not requiring anti-magnetic devices.

Note that while the present invention was explained heretofore withreference to the case of application to a speaker, the present inventionmay be applied to all sound generating devices for producing sound, suchas telephones or buzzers. Further, needless to say, unimorphs may beused as the piezoelectric vibration plates.

I claim:
 1. A sound generating device comprising:an acoustic vibrationplate; a driving device having a plurality of piezoelectric vibrationplates, the central portion of at least one of the piezoelectricvibration plates being connected to said acoustic vibration plate fordriving said acoustic vibration plate, wherein said piezoelectricvibration plates are arranged spaced from each other in an axialdirection, wherein one of the peripheral and central portions ofadjacent piezoelectric vibration plates are connected to each other,adjacent piezoelectric vibration plates being made to flex in oppositedirections from each other, the piezoelectric vibration plate positionedat one end of said plurality of piezoelectric vibration plates beingconnected to an acoustic vibration plate; and an elastic plate memberattached to the peripheral portion of said piezoelectric vibration plateand radially extending outward from the periphery of the piezoelectricvibration plate, said elastic plate member being attached to theperipheral portion of said piezoelectric vibration plate positioned atthe other end of said plurality of piezoelectric vibration plates.
 2. Asound generating device as set forth in claim 1, wherein said drivingmember has at least one module comprised of a pair of piezoelectricvibration plates connected to each other at peripheral portions thereof.3. A sound generating device as set forth in claim 2, wherein thecentral portion of one piezoelectric vibration plate of said pair ofpiezoelectric vibration plates is connected to an acoustic vibrationplate through a connecting rod.
 4. A sound generating device as setforth in claim 2, wherein said driving device is provided with aplurality of modules connected in series.
 5. A sound generating deviceas set forth in claim 4, wherein the central portions of thepiezoelectric vibration plates of said modules are connected to eachother through connecting rods.
 6. A sound generating device as set forthin claim 2, wherein said driving device is provided with a singlepiezoelectric vibration plate arranged adjacent to said module, thecentral portion of said single piezoelectric vibration plate isconnected to the central portions of one piezoelectric vibration plateconstituting the module, and the peripheral portion of said singlepiezoelectric vibration plate is connected to the acoustic vibrationplate.
 7. A sound generating device as set forth in claim 6, wherein thecentral portion of said single piezoelectric vibration plate and thecentral portion of said one piezoelectric vibration plate are connectedto each other through a connecting rod.
 8. A sound generating device asset forth in claim 6, wherein the central portion of said singlepiezoelectric vibration plate and the central portion of said onepiezoelectric vibration plate are connected to each other through ahollow sleeve communicating an air damper chamber with the outside airand wherein said hollow sleeve is closed by a plug so as to shut off theair damper chamber from the outside air.
 9. A sound generating device asset forth in claim 2, wherein the peripheral portions of said pair ofpiezoelectric vibration plates are connected to each other by an annularspacer extending along the peripheral portions of the piezoelectricvibration plates and wherein an air damper chamber for attenuation ofhigh frequency oscillation enclosed by said annular spacer is formedbetween said pair of piezoelectric vibration plates.
 10. A soundgenerating device as set forth in claim 9, wherein said driving memberis provided with a plurality of modules, the central portions of thepiezoelectric vibration plates of mutually adjoining modules areconnected to each other by hollow sleeves, and air damper chambersformed in the modules are connected to each other through said hollowsleeves.
 11. A sound generating device as set forth in claim 9, whereinthrough holes for communicating the inside of the air damper chamberwith the outside air are formed in at least one of said annular spacerand piezoelectric vibration plates.
 12. A sound generating device as setforth in claim 9, wherein said annular spacer is formed from a metalmaterial.
 13. A sound generating device as set forth in claim 9, whereinsaid annular spacer is formed from an elastic material.
 14. A soundgenerating device as set forth in claim 13, wherein said elasticmaterial is comprised of a rubber.
 15. A sound generating devicecomprising:an acoustic vibration plate; a driving device having aplurality of piezoelectric vibration plates, the central portion of atleast one of the piezoelectric vibration plates being connected to saidacoustic vibration plate for driving said acoustic vibration plate,wherein said driving device is comprised of a pair of piezoelectricvibration plates connected to each other at central portions thereof,the peripheral portion of one of said piezoelectric vibration platesbeing connected to said acoustic vibration plate; and an elastic platemember attached to the peripheral portion of said piezoelectricvibration plate and radially extending outward from the periphery of thepiezoelectric vibration plate, said elastic plate member being attachedto the peripheral portion of the other piezoelectric vibration plate.16. A sound generating device comprising:a frame: an acoustic vibrationplate; a driving device having at least one piezoelectric vibrationplate, wherein a peripheral portion of said at least one piezoelectricvibration plate is connected to said acoustic vibration plate fordriving said acoustic vibration plate; and a flexible, elastic platemember attached to a central portion of said piezoelectric vibrationplate via a connecting rod and radially extending outward from theconnecting rod, wherein the elastic plate member is coupled to the frameonly via the central portion of the piezoelectric-vibration plate sothat a peripheral portion of the elastic plate member is freely movablewith respect to the frame.
 17. A sound generating device as set forth inclaim 16, wherein said at least one piezoelectric vibration platesincludes a plurality of piezoelectric vibration plates which arearranged spaced from each other in an axial direction, one of theperipheral portions and the central portions of adjacent piezoelectricvibration plates being connected to each other, adjacent piezoelectricvibration plates being made to flex in opposite directions from eachother, wherein the piezoelectric vibration plate positioned at a firstend of said plurality of piezoelectric vibration plates is connected tothe acoustic vibration plate, said elastic plate member being attachedto the central portion of said one of the piezoelectric vibration platespositioned at a second end of said plurality of piezoelectric vibrationplates.
 18. A sound generating device as set forth in claim 17, whereinsaid sound generating device is provided with a frame, and a peripheralportion of the elastic plate member is supported by said frame.
 19. Asound generating device as set forth in claim 17, wherein said drivingdevice is provided with a single piezoelectric vibration plate arrangedadjacent to said module, a central portion of said single piezoelectricvibration plate being connected to the central portion of one of thepiezoelectric vibration plates constituting the module, and wherein theperipheral portion of said single piezoelectric vibration plate isconnected to the acoustic vibration plate.
 20. A sound generating deviceas set forth in claim 19, wherein the central portion of said singlepiezoelectric vibration plate and the central portion of said one of thepiezoelectric vibration plates constituting the module are connected toeach other through a connecting rod.
 21. A sound generating device asset forth in claim 19, wherein the central portion of said singlepiezoelectric vibration plate and the central portion of said onepiezoelectric vibration plate are connected to each other through ahollow sleeve communicating an air damper chamber with the outside airand wherein said hollow sleeve is closed by a plug so as to shut off theair damper chamber from the outside air.
 22. A sound generating deviceas set forth in claim 16, wherein said elastic plate member is made of arubber material.
 23. A sound generating device as set forth in claim 16,wherein said at least one piezoelectric vibration plates includes,wherein the at least one piezoelectric vibration plate comprises atleast one module comprised of a pair of piezoelectric vibration platesconnected to each other at peripheral portions thereof.
 24. A soundgenerating device as set forth in claim 23, wherein the central portionof one of said piezoelectric vibration plates of the at least one moduleis connected to said acoustic vibration plate, said elastic plate memberbeing attached to the central portion of the other one of thepiezoelectric vibration plates of the at least one module.
 25. A soundgenerating device as set forth in claim 23, wherein the peripheralportions of said pair of piezoelectric vibration plates of the at leastone module are connected to each other by an annular spacer extendingalong the peripheral portion of each of the piezoelectric vibrationplates and wherein an air damper chamber for attenuation of highfrequency oscillation enclosed by said annular spacer is formed betweensaid pair of piezoelectric vibration plates.
 26. A sound generatingdevice as set forth in claim 25, wherein said driving member is providedwith a plurality of modules and wherein central portions of thepiezoelectric vibration plates of mutually adjoining modules areconnected to each other by hollow sleeves, and wherein air damperchambers formed within each of the modules are connected to each otherthrough said hollow sleeves.
 27. A sound generating device as set forthin claim 25, wherein through holes for communicating the inside of theair damper chamber with the outside air are formed in at least one ofsaid annular spacer and said piezoelectric vibration plates.
 28. A soundgenerating device as set forth in claim 25, wherein said annular spaceris formed from a metal material.
 29. A sound generating device as setforth in claim 25, wherein said annular spacer is formed from an elasticmaterial.
 30. A sound generating device as set forth in claim 29,wherein said elastic material is comprised of a rubber material.
 31. Asound generating device-as set forth in claim 23, wherein said drivingdevice is provided with a plurality of modules connected in series. 32.A sound generating device as set forth in claim 31, wherein the centersof the piezoelectric vibration plates-of said modules are connected toeach other through connecting rods.
 33. A sound generating device as setforth in claim 16, wherein said piezoelectric vibration plate iscomprised of a bimorph.
 34. A sound generating device comprising:aframe; an acoustic vibration plate; a driving device having at least onepiezoelectric vibration plate, the central portion of said piezoelectricvibration plate being connected to said acoustic vibration plate fordriving said acoustic vibration plate; and an elastic, flexible platemember, a central portion of which is attached to the peripheral portionof said piezoelectric vibration plate, wherein the elastic plate memberextends radially outward from the periphery of the piezoelectricvibration plate and wherein the elastic plate member is coupled to theframe only via the piezoelectric vibration plate so that a peripheralportion of the elastic plate member is freely movable with respect tothe frame.
 35. A sound generating device as set forth in claim 1,wherein said sound generating device is provided with a frame, and theperipheral portion of the elastic plate member is supported by saidframe.
 36. A sound generating device as set forth in claim 34, whereinsaid elastic plate member is made of a rubber.
 37. A sound generatingdevice as set forth in claim 34, wherein a plurality of driving devicesare provided in parallel with respect to the acoustic vibration plate soas to drive the acoustic vibration plate.
 38. A sound generating deviceas set forth in claim 34, wherein said piezoelectric vibration plate iscomprised of a bimorph.